This proposal investigates the effects of recurrent hypoglycemia on glucose sensing neurons in the ventromedial hypothalamic nucleus (VMN). The overall objective is to gain a clearer understanding of (a) how changes in glucose are transduced into neuronal signals, and (b) how those neuronal signals are integrated to initiate the counterregulatory response to hypoglycemia. The two hypotheses in this proposal directly address these issues. HYPOTHESIS I is that VMN glucose sensing neurons have the potential to sense impending hypoglycemia and signal for the initiation of the counterregulatory response. This hypothesis derives from our knowledge of the ability of VMN glucose sensing neurons to respond to physiologically relevant changes in extracellular glucose. HYPOTHESIS II is that VMN glucose sensing neurons receive information from and project to brain regions important to the regulation of glucose homeostasis. The first set of experiments proposed herein will define the responsiveness of physiologically relevant subtypes of VMN glucose sensing neurons over physiological glucose concentrations. We will then determine whether or how the responses are altered by recurrent hypoglycemia sufficient to induce hypoglycemia associated autonomic failure. We will also explore the cellular mechanism of glucose sensing transduction so that the mechanisms that underlie any change in responsiveness can be understood. The second set of experiments will provide the basis for understanding the potential integrative role of VMN glucose sensing neurons. This will be accomplished by identifying presynaptic inputs and transmitters to the VMN from other glucose sensing neurons and identifying the hypothalamic targets of VMN glucose sensing neurons. These studies will provide important information regarding the way that the brain regulates glucose homeostasis and the way that perturbations in glucose homeostasis alter these central mechanisms. This will facilitate the development of new and effective treatments for Type I diabetes mellitus and its major modern complication hypoglycemia.